The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit is generally the 2T1C, i.e. the structure of two thin film transistors plus one capacitor to convert the voltage into the current. However, the threshold voltage of the traditional 2T1C pixel driving circuit will drift along with the working times. Thus, it results in that the luminescence of the OLED is unstable and the nonuniform luminescence and uneven brightness among the respective pixels.
The main method of solving the AMOLED uneven brightness is to improve the pixel driving circuit, and to add the compensation function to make the influence of the threshold voltage variation of the drive thin film transistor to the current flowing through the organic light emitting diode be smaller.
As shown in FIG. 1, which shows an AMOLED pixel driving circuit according to prior art, The AMOLED pixel driving circuit has the 5T2C structure, i.e. the structure of five thin film transistors plus two capacitors, comprising: a first thin film transistor T10, a second thin film transistor T20, a third thin film transistor T30, a fourth thin film transistor T40, a fifth thin film transistor T50, a first capacitor C10, a second capacitor C20 and an organic light emitting diode D10, and all the respective thin film transistors are P type thin film transistors. Specifically, the first thin film transistor T10 is a drive thin film transistor, and a gate thereof is electrically coupled to one end of the first capacitor C10 through a first node A0, and a source is electrically coupled to a power source positive voltage VDD, and a drain is electrically coupled to a source of a fifth thin film transistor T50; a gate of the second thin film transistor T20 receives a scan signal SCAN, and a source receives a data signal data, and a drain is electrically coupled to the other end of the first capacitor C10 through a second node B0; a gate of the third thin film transistor T30 receives a light emitting control signal EM, and a source receives a reference voltage Vref, and a drain is electrically coupled to the second node B0; a gate of the fourth thin film transistor T40 receives the scan signal SCAN, and a source is electrically coupled to the first node A0, and a drain is electrically coupled to the drain of the first thin film transistor T10 and a source of the fifth thin film transistor T50; a gate of the fifth thin film transistor T50 receives the light emitting control signal EM, and a source is electrically coupled to the drain of the first thin film transistor T10 and the drain of the fourth thin film transistor T40, and a drain is electrically coupled to an anode of the organic light emitting diode D10; the one end of the first capacitor C10 is electrically coupled to the first node A0, and the other end is electrically coupled to the second node B0; one end of the second capacitor C20 is electrically coupled to the first node A0, and the other end is electrically coupled to the power source positive voltage VDD; the anode of the organic light emitting diode D10 is electrically coupled to the drain of the fifth thin film transistor T50, and a cathode is electrically coupled to a power source negative voltage VSS.
FIG. 2 is a sequence diagram of the AMOLED pixel driving circuit of the 5T2C structure according to prior art as shown in FIG. 1, and the working process of the AMOLED pixel driving circuit can be divided into four stages according to the sequence: an initializing stage 10, a threshold voltage sampling stage 20, a holding stage 30 and a drive stage 40. With combination of FIG. 2 and FIG. 3, in the initializing stage 10, the scan signal SCAN provides low voltage level, and the second thin film transistor T20 and the fourth thin film transistor T40 controlled by the scan signal SCAN are activated, and the light emitting control signal EM provides low voltage level, and the third thin film transistor T30 and the fifth thin film transistor T50 controlled by the light emitting control signal EM are activated, and the data signal data is transmitted to the second node B0 through the second thin film transistor T20, and charges the first capacitor C10 to make the voltage of the second node B0 to be the data signal voltage Vdata, and in this stage, both the fourth thin film transistor T40 and the fifth thin film transistor T50 are activated, and the voltage of the first node A0, i.e. the gate voltage of the first thin film transistor T10 Vg=VOLED, and VOLED is the anode voltage of the organic light emitting diode D10. With combination of FIG. 2 and FIG. 4, in the threshold voltage sampling stage 20, the scan signal SCAN still provides low voltage level, and the light emitting control signal EM is raised from low voltage level to the high voltage level, and the third thin film transistor T30 and the fifth thin film transistor T50 are deactivated, and the voltage of the first node A0, i.e. the gate voltage Vg of the first thin film transistor T10 is changed to be VDD−Vth, and Vth is the threshold voltage of the first thin film transistor T10. With combination of FIG. 2 and FIG. 5, in the holding stage 30, the scan signal SCAN is raised from low voltage level to the high voltage level, and the light emitting control signal EM is kept to be high voltage level, and the second thin film transistor T20 and the fourth thin film transistor T40 are deactivated, and under the coupling function of the first capacitor C10 and the second capacitor C20, the voltages of the first node A0 and the second node B0 are raise with ΔV, and correspondingly, the gate voltage of the first thin film transistor T10 is Vg=VDD−Vth+ΔV. With combination of FIG. 2 and FIG. 6, in the drive stage 40, the light emitting control signal EM is dropped from high voltage level to the low voltage level, and the scan signal SCAN remains to be high voltage level, and the third thin film transistor T30 and the fifth thin film transistor T50 are activated, again, and the organic light emitting diode D10 starts to emit light, and then the voltage of the first node A0, i.e. the gate voltage of the first thin film transistor T10 is Vg=VDD−Vth+ΔV+Vref−Vdata, and the reference voltage Vref makes the voltage of the second node B0 drop to Vref through the activated third thin film transistor T30, and the source voltage Vs of the first thin film transistor T10 is VDD the same in the respective stages, and in the drive stage, the gate-source voltage of the first thin film transistor T10 is Vgs=Vg−Vs=VDD−Vth+ΔV+Vref−Vdata−VDD. Furthermore, as known, the formula of calculating the current flowing through the organic light emitting diode as the drive thin film transistor is a P type thin film transistor is:IOLED=½Cox(μW/L)(Vgs+Vth)2 
wherein IOLED is the current of the organic light emitting diode D10, μ is the carrier mobility of the drive thin film transistor, i.e. the first thin film transistor T10, and W and L respectively are the width and the length of the channel of the drive thin film transistor, i.e. the first thin film transistor T10, and Vgs is the gate-source voltage of the drive thin film transistor, i.e. the first thin film transistor T10, and Vth is the threshold voltage of the drive thin film transistor, i.e. the first thin film transistor T10.
Vgs=VDD−Vth+ΔV+Vref−Vdata−VDD is substituted into the current calculation formula, and then:IOLED=½Cox(μW/L)(ΔV+Vref−Vdata)2 
Obviously, the current flowing through the organic light emitting diode D10 is irrelevant with the threshold voltage Vth of the first thin film transistor T10, and the present AMOLED pixel driving circuit realizes the compensation function.
However, the present AMOLED pixel driving circuit requires setting the two signals, the scan signal and the light emitting control signal to control the corresponding thin film transistors. The amount of the signal lines is increased to raise the loading of the control IC, which goes against the saving of the cost.